Paving a way to treat spastic paraplegia 50

Spastic paraplegia 50 (SPG50) is a rare neurodegenerative disease caused by loss-of-function mutations in AP4M1. There are no effective treatments for SPG50 or any other type of SPG, and current treatments are limited to symptomatic management. In this issue of the JCI, Chen et al. provide promising data from preclinical studies that evaluated the efficacy and safety profiles of an AAV-mediated AP4M1 gene replacement therapy for SPG50. AAV/AP4M1 gene replacement partly rescued functional defects in SPG50 cellular and mouse models, with acceptable safety profiles in rodents and monkeys. This work represents a substantial advancement in therapeutic development of SPG50 treatments, establishing the criteria for taking AAV9/AP4M1 gene therapy to clinical trials.

Notably, therapeutic development for HSP largely lags behind its comprehensive genetic and mechanistic studies. There are no effective treatments to prevent, slow, or reverse HSP. Current treatment of HSP is limited to symptomatic relief (2).
Spastic paraplegia 50 (SPG50) is a rare and severe form of complex HSP. It is characterized by early infantile hypotonia, developmental delay, intellectual disability, and microcephaly. Over time, hypotonia evolves into spasticity of the lower limbs and then upper limbs, resulting in spastic tetraplegia. SPG50 is transmitted as an autosomal recessive disease caused by loss-of-function mutations in AP4M1, which encodes for the medium subunit of the adaptor protein complex 4 (AP-4) (3,4). AP-4 is composed of four subunits, including two large chains (AP4B1 and AP4E1), a medium chain (AP4M1), and a small chain (AP4S1). Notably, biallelic loss-of-function mutations in each of these four AP-4 subunits lead to clinically overlapping HSP types, including SPG47 (AP4B1), SPG50 (AP4M1), SPG51 (AP4E1), and SPG52 (AP4S1), which could be collectively called "AP-4-associated HSP" or "AP-4 deficiency syndrome" (5-7). In mammals, there are five AP complexes (AP-1 to AP-5) that facilitate the selective sorting of transmembrane cargo proteins into vesicles and mediate related intracellular vesicle trafficking. AP-4 primarily localizes to the trans-Golgi network (TGN) and has been shown to mediate protein trafficking from the TGN to other cellular compartments, such as the endosomallysosomal system (8). Although the pathogenic mechanism by which loss of AP4M1 leads to SPG50 remains unclear, putting the missing AP4M1 back into the affected cells is presumably the most rational option for SPG50 therapeutic development.

Gene replacement therapy for SPG50
In this issue of the JCI, Chen et al. (9) provide promising data from preclinical studies that evaluated the efficacy and safety profiles of an AAV-mediated AP4M1 gene replacement therapy for SPG50. The authors used a self-complementary AAV (scAAV) vector carrying the human AP4M1 gene under the control of the ubiquitous synthetic promotor (UsP) (AAV/AP4M1). Transduction of the primary fibroblasts from patients with SPG50 with AAV2/AP4M1 vector rescued the cellular phenotypes of AP4M1 deficiency, as shown by restoration of ATG9A trafficking from the TGN and by increased levels of AP4E1. To test the therapeutic potential in vivo, the authors administered AAV9/AP4M1 to Ap4m1-KO mice by intrathecal injection. They observed dose-dependent increased levels of AP4M1 mRNA in the brain regions, and partial restoration of the abnormal behavioral phenotypes, most robustly shown by the hind limb clasping test in the Ap4m1-KO mice ( Figure 1). No apparent adverse effects were observed in the AAV9/AP4M1-treated Ap4m1-KO mice up to 20 months after injection.
The authors evaluated the safety profiles of the AAV9/AP4M1 intrathecal administration further in normal mice, rats, and monkeys. They found that the Spastic paraplegia 50 (SPG50) is a rare neurodegenerative disease caused by loss-of-function mutations in AP4M1. There are no effective treatments for SPG50 or any other type of SPG, and current treatments are limited to symptomatic management. In this issue of the JCI, Chen et al. provide promising data from preclinical studies that evaluated the efficacy and safety profiles of an AAV-mediated AP4M1 gene replacement therapy for SPG50. AAV/AP4M1 gene replacement partly rescued functional defects in SPG50 cellular and mouse models, with acceptable safety profiles in rodents and monkeys. This work represents a substantial advancement in therapeutic development of SPG50 treatments, establishing the criteria for taking AAV9/AP4M1 gene therapy to clinical trials. J Clin Invest. 2023;133(10):e170226 https://doi.org/10.1172/JCI170226 is prominent in the mice (10). This deficit may be partly due to the anatomical and functional differences of the corticospinal tracts between humans and rodents (11,12). Second, dose-related neuronal toxicities, such as dorsal root ganglion degeneration, were observed in rats and monkeys, indicating the need for a lower dose range in clinical trials. Although a lower dose range will presumably reduce the toxicities, it may also limit the therapeutic efficacy. Enhancing AP4M1 expression with a stronger promoter will likely reduce the AAV dose and thus minimize its related toxicities. The authors used UsP promoter to drive AP4M1 expression. UsP is a short promoter (328 bp), which can be easily accommodated in the scAAV vector that has a size limit of approximately 2.3 kb. But UsP is a relatively weaker promoter, with about 15% activity of CMV promoter (13). Notably, a larger but stronger hybrid CMV enhancer/chicken

Conclusions and implications
Chen et al. (9) have provided a comprehensive preclinical study that explores gene replacement as a therapeutic strategy for HSP. However, there are some intrinsic limitations. First, the Ap4m1-KO mice did not show robust and reliable motor phenotypes, even though patients with SPG50 eventually develop severe spastic tetraplegia. This inability to fully recapitulate the symptoms makes it challenging to effectively evaluate therapeutic efficacy. The improvement of phenotypes in Chen et al. (9) was primarily based on the hind limb clasping test. The improvements in other measures were variable, inconsistent with the doses, and thus difficult to interpret. Nevertheless, lacking robust motor phenotypes is not specific to the SPG50 mouse model, as no SPG mouse models to date display robust motor phenotypes, even though the degenerative pathology of the corticospinal tracts AAV9/AP4M1 gene therapy was overall safe and well tolerated in these animals. AAV9/AP4M1 vector did not generate a detectable T cell immune response to either AAV9 or the human AP4M1 protein. Infrequent occurrence of hepatocellular adenomas was observed in some of the one-yearold mice, but this is unlikely related to the AAV9/AP4M1 treatment. Minimal-to-mild neuronal toxicities were observed in rats and monkeys in three-month toxicology studies, which included neuronal degeneration noted microscopically in the lumbar dorsal root ganglion in both rats and monkeys. In the monkeys, neuronal toxicities were also observed in other regions of the central and peripheral nervous systems. Notably, these toxicities were dose related, as they preferentially occurred in the higher-dose cohorts, suggesting that dose optimization is required for the future clinical trials. β-actin promoter is used in a similar AAV9based vector, called onasemnogene abeparvovec, for treating spinal muscular atrophy. But the much larger protein size of AP4M1 (453 amino acids) compared with that of SMN1 (262 amino acids) prevents the use of chicken β-actin or other much larger promoters to drive AP4M1 expression in the scAAV9 vector due to its size limit. Thus, the balance between therapeutic efficacy and toxicity may need further optimization in clinical trials.
The findings by Chen et al. (9) show promising evidence that AAV9/AP4M1 gene replacement can partly rescue functional defects in the SPG50 cellular and mouse models with acceptable safety profiles in the rodents and monkeys when the dose is adjusted. Even though the authors state the need for further inquiry, the study by Chen et al. (9) provides critical preclinical data necessary to push AAV9/AP4M1 gene therapy to clinical trials. This study represents a substantial advance in therapeutic development, particularly when considering the poor prognosis of SPG50.